In typical polymer electrolyte membrane (PEM) fuel cells, a membrane electrode assembly (MEA) is disposed between two electrically conductive separator plates. Oxidant and fuel flow fields provide means for directing the oxidant and fuel to respective electrocatalyst layers of the MEA, specifically, to an anode on the fuel side and to a cathode on the oxidant side of the MEA. A typical reactant fluid flow field has at least one channel through which a reactant stream flows. The fluid flow field is typically integrated with the separator plate by locating a plurality of open-faced channels on one or both faces of the separator plate. The open-faced channels face an electrode, where the reactants are electrochemically reacted. Typically more reactant is supplied to the electrodes than is actually consumed by the electrochemical reactions in the fuel cell. Stoichiometry (or stoichiometry ratio) can be defined as the molar flow rate of a reactant supplied to a fuel cell divided by the molar flow rate of reactant consumed in the fuel cell; reactant stoichiometry is the inverse of reactant utilization.
In a single cell arrangement, separator plates are provided on each of the anode and cathode sides. The plates act as current collectors and provide structural support for the electrodes. In a fuel cell stack, often bipolar plates, having reactant flow fields on both sides, are interleaved with MEAs.
In conventional fuel cell flow fields, the reactant flow channels have a constant cross-sectional area and shape along their length. Typically the channels are square or rectangular in cross-section. However, fuel cells where the cross-sectional area of flow channels varies along their length are known. For example, Applicant's issued U.S. Pat. No. 6,686,082 and U.S. Patent Application Publication No. US2006/0234107 (each of which is hereby incorporated by reference herein in its entirety) describe fuel cell flow field plates with a trapezoidal form, in which the flow channels are rectangular in cross-section and have a cross-sectional area that continuously diminishes in the flow direction. In particular, embodiments are described in which the flow channel width decreases linearly in the flow direction. Such tapered channels provide an increase in reactant velocity along the channel, and were intended to provide some of the advantages of serpentine flow channels without the significant pressure drop and the related increase in parasitic load usually associated with serpentine flow channels. Serpentine flow fields provide higher reactant velocity and improved water removal in the channels relative to conventional flow fields covering roughly the same flow field area.
It is also known that it is possible to enhance fuel cell performance by varying the cross-sectional area of cathode flow channels along their length. Fuel cells generally operate on a dilute oxidant stream, namely air, at the cathode. As the air flows along the cathode flow channel(s), the oxygen content in the air stream tends to be depleted and the air pressure tends to drop. This can result in reduced performance and uneven current density in the fuel cell. Applicant's issued U.S. Pat. No. 7,838,169 (which is incorporated herein by reference in its entirety) describes improved cathode flow field channels, with a more sophisticated variation in channel cross-section, which can be used to achieve substantially constant oxygen availability at the cathode. Embodiments are described in which the cross-sectional area of the channels varies along the length of the channels such that oxygen availability is kept substantially constant for a given channel length and air stoichiometry. In some embodiments the channel width decreases in the oxidant flow direction according to an exponential function. By maintaining substantially constant oxygen availability along the channel, use of such cathode flow channels has been shown to provide improved uniformity of current density and to enhance fuel cell performance.
There are a number of factors that can lead to irreversible performance losses and/or loss of electrochemical surface area during prolonged operation in PEM fuel cells. The losses are mainly the result of catalyst layer degradation including platinum metal dissolution and corrosion of carbon supports. Carbon corrosion can be caused by cell reversal which, in turn, can be a result of uneven reactant distribution and/or fuel starvation in regions of the anode. Platinum dissolution can be caused by high potentials at the cathode or anode which can occur when there is heterogeneous current distribution. Individually or coupled together, these degradation rates can be significant and require mitigation for long term stable operation. Operating a fuel cell with uniform current density can reduce carbon corrosion and platinum dissolution, both of which are detrimental to fuel cell performance, longevity and durability. It can be postulated that achieving more uniform current density will reduce fuel cell degradation rates and thereby improve fuel cell lifetime. Furthermore, increasing velocities down the channel can reduce the residence time of hydrogen/air fronts during air purge cycles at the anode during fuel cell start-stop sequences, leading to reduced carbon corrosion rates and improved durability.
The flow field channels described in the '169 patent were originally developed for and applied at the fuel cell cathode, and were not expected to provide particular benefits at the anode, at least in part because fuel cell operation on substantially pure hydrogen does not result in substantial mass transport losses due to high concentration and the high diffusivity of the hydrogen molecule. However, Applicants have now determined that such flow field channels can be modified for use at the anode to enhance fuel cell performance when the fuel cell is operating on a dilute fuel, and to provide performance benefits even when the fuel cell is operating on a substantially pure fuel.
Often a substantially pure fuel stream (such as hydrogen) is used at the fuel cell anode, and the issue of reactant depletion and reactant availability along the anode flow channels is not the same as with air at the cathode. Usually, for operation on a substantially pure fuel, the anode flow field is “dead-ended”, and the fuel cell or fuel cell stack is fitted with a bleed- or purge-valve for removing impurities, contaminants and/or water that tends to accumulate in the downstream portion of the anode flow channels as fuel is consumed during operation of the fuel cell. This accumulation of “inerts” can result in uneven current density, resulting in reduced fuel cell performance. Furthermore, accumulation of water in the anode flow channels can cause other problems including blocking fuel access to the anode catalysts leading to a fuel starvation condition that has been linked to both carbon corrosion and noble metal catalyst dissolution. Applicants have determined that flow field channels similar to those described in the '169 patent can be modified for use at the anode to reduce some of these detrimental effects at the anode, and enhance fuel cell performance even on a substantially pure fuel. Such channels can provide an increase in the velocity of the fuel during its passage along the channel. It is believed that this can improve fuel utilization, as increasing the flow velocity on the anode can provide a higher fuel availability leading to a leveling of the current density at lower flow rates. This can reduce fuel consumption for a given power output. Furthermore, if the fuel cell is operating on a dilute fuel stream (such as a reformate stream which contains hydrogen, or aqueous methanol), as the fuel stream flows along the flow channels, the hydrogen content in the stream tends to be depleted and the fuel stream pressure tends to drop. Both of these can result in reduced fuel cell performance. The anode flow field channels described herein can be used to compensate for the depletion of fuel, achieving substantially constant fuel availability on a dilute fuel, and thereby providing more uniform current density.
In addition to their modification for use at the anode, further improvements and variations on the flow field channels and flow fields described in the '169 patent have been made and are described herein. In particular, embodiments in which such flow channels can be incorporated or retrofit into conventional rectangular fuel cell geometries at the anode and/or cathode (rather than trapezoidal fuel cells) with high utilization of the fuel cell active area and efficient use of MEA and plate materials are described. Embodiments are also described in which the characteristics of the channel vary as a function of distance along the channel in accordance with the same or similar principles as in the '169 patent and as described herein, but along only a portion of the channel length. Further embodiments are described in which the characteristics of the channel vary as a function of distance along the channel in accordance with the same or similar principles, but in a stepwise or discontinuous manner. Such embodiments can be used to achieve at least some of the performance benefits described above, and can also provide options for improved flow fields that are easier to fabricate or to incorporate into rectangular fuel cell plate geometries.